1. Field of the Invention
The present invention relates to an information recording and reproducing apparatus and, more particularly, to an information recording and reproducing apparatus of the hard disk type capable of avoiding off-track error.
2. Description of the Prior Art
A conventional information recording and reproducing apparatus of the hard disk type (hereinafter referred to simply as "recording and reproducing apparatus") is shown in FIGS. 5 and 6. The recording and reproducing apparatus comprises, as principal components, magnetic disks 1, magnetic heads 2 for recording information in the disks 1 and reproducing the information recorded in the magnetic disks 1, a motor 3 for rotatively driving the magnetic disks 1, a magnetic disk driving mechanism 4 for shifting the magnetic heads 2 to a predetermined track of the corresponding magnetic disk 1, a housing 5 accommodating the magnetic disks 1, the magnetic heads 2, the motor 3 and the magnetic head driving mechanism 4, and a top cover for covering up the housing 5.
In this recording and reproducing apparatus, four magnetic heads 2 are provided each for one side of each of two magnetic disks 2 to record information on both sides of each magnetic disk 1. Each magnetic head 2 is attached to the free end of a support spring 7 attached to the free end of a swing arm 8 of the magnetic head driving mechanism 4. The magnetic head driving mechanism 4 comprises the swing arm 8, a steel belt 9 attached to the base end of the swing arm 8, namely, one end of the swing arm 8 opposite the other end supporting the magnetic head 2, a pulley 10 engaging the intermediate portion of the steel belt 9, and a step motor 11. The pulley 10 is secured to the output shaft 12 of the step motor 11. The step motor 11 drives the swing arm 8 for swing motion on a pivot shaft 8a.
A case accommodating the magnetic disks 1, the magnetic heads 2, the swing arm 8, the steel belt 9, the pulley 10 and the associated parts consists of the housing 5 and the top cover, not shown. Gaskets are provided between the contact surfaces of the housing 5 and the top cover and between the step motor 11 and a step motor mounting part and the journal box of the motor 3 is filled with a magnetic fluid to seal the case hermetically. An opening is formed in part of the housing 5 as a breather for maintaining equality of pressure within and without the airtight enclosure of the case and the breather is provided with a filter 13.
As illustrated in FIG. 8, the motor 3 for driving the magnetic disks 1 is a direct drive DC brushless motor. The rotor unit 15 of the motor 3, having permanent magnets 14 protrudes from the underside of the housing 5. A flange 16 provided with a boss is disposed over the rotor unit 15 and a driving coil 17 is provided on the underside of the flange 16. Bearings 19 and 20 for rotatably supporting a driving shaft 18 are fitted in the boss of the flange 16. The flange 16 is fitted tightly in a hole formed in the housing 5 and is screwed to the housing 5. As illustrated in FIG. 9, a shallow groove 27 for passing air is formed in the upper surface of the flange 16 so as to extend from the circumference to a position 25 corresponding to an aperture 24 formed in the flange of a driving hub 21 which will be described hereunder.
The driving hub 21 is secured to the driving shaft 18 of the motor 3. The driving hub 21 consists of a cylindrical body 28 and a flange 22 formed at the lower end of the cylindrical body 28, as illustrated in FIG. 8. A plurality of longitudinal grooves 23 are formed in the outer surface of the cylindrical body 28 so as to extend from the top surface 29 of the cylindrical body 28 through openings 24 formed in the flange 22 to the underside of the flange 22, respectively. Recesses 30 are formed in the upper surface of the flange 22 so as to extend from the corresponding grooves 23 to the circumference of the flange 22, respectively. The cylindrical surfaces 31 of the cylindrical body 28 and the upper surface of the flange 22 serve as reference surfaces for positioning the disks 1. The cylindrical surfaces 31 and the upper surface of the flange 22 are finished at a high accuracy with reference to the seating surface 35 of the flange 16 after attaching the driving hub 21 to the driving shaft 18.
A spacer ring 36 is fitted on the driving hub 21 between the two magnetic disks 1. A plurality of recesses 37 are formed in the upper and lower surfaces of the spacer ring 36 so as to extend from the inner circumference to the outer circumference of the spacer ring 36.
A practically disk-shaped clamp 38 having a plurality of radially expanding recesses 39 in the lower surface thereof is fastened to the upper surface 29 of the driving hub 21 with screws 38a to the upper surface 29 of the driving hub 21 to hold the magnetic disks 1 and the spacer ring 36 firmly in place on the driving hub 21.
The motor 3 is fitted in a hole 5a formed in the housing 5; the driving hub 21 is attached to the driving shaft 18 of the motor 3; and the magnetic disks 1 are fitted on and secured to the cylindrical body 28 of the driving hub 21. Thus, the disks 1 are rotated within the housing 5.
The swing arm 8 supporting the magnetic heads 2 is swingable on the pivot shaft 8a. As illustrated in FIGS. 6 and 7, the pivot shaft 8a is supported rotatably on two bearings, namely, upper and lower bearings as viewed in FIGS. 6 and 7, 8b and 8c fitted in a bore 5b formed in the housing 5. The bore 5b receives therein a pivot assembly 8f comprising the pivot shaft 8a, the bearings 8b and 8c, a preloading spring 8d for urging the bearings 8b and 8c away from each other and an E-ring 8e for retaining the bearing 8c in place. The tolerance of fit for fitting the bearing 8b in the bore 5b is in the range of 20 to 30 .mu.m. The clearance between the outer circumference of the bearing 8b and the inner surface of the bore 5b is filled with an adhesive or the like to fix the bearing 8 b in the bore 5b. The lower bearing 8c is not fixed.
In this conventional recording and reproducing apparatus, the housing 5 is made of an aluminum alloy through die casting; the bearings 8b and 8c supporting the pivot shaft 8 are made of a stainless steel; the flange 16 of the motor 3 is made of an aluminum alloy through die casting; and the bearings 19 and 20 supporting the driving shaft 18 are made of a high-carbon-chromium bearing steel. That is, the bearings 8b, 8c, 19 and 20 supporting rotary members are made of a stainless steel or a high-carbon-chromium bearing steel, while the members holding these bearings 8b, 8c, 19 and 20 are made of an aluminum alloy. Since the coefficient of thermal expansion of aluminum alloys is greater than those of stainless steels and high-carbon-chromium steels, clearance develops between the housing 5 and the bearings 8b and 8c and between the flange 16 and the bearings 19 and 20, when the temperature of these components rises. When such clearance develops, the driving shaft 18 of the motor 3 and the pivot shaft 8a on which the swing arm 8 turns tilt in random directions, so that off-track occurs, and thereby the recording and reproducing characteristics of the recording and reproducing apparatus are deteriorated. For example, if the radii of the bearings 8b and 8c supporting the pivot shaft 8 a are 6.5 mm and the temperature of these bearings 8b and 8c is raised by 30.degree. C., the horizontal dislocation of the pivot shaft 8a is approximately 2 .mu.m.